The invention relates to a mobile cellular telephone system having the capability of transmitting digital data without errors caused by handoff operations, power adjusting operations, multipath fading, and the like.
Mobile cellular radiotelephone service is readily available. A cellular system has a coverage area divided into contiguous smaller coverage "cells" using low power transmitters and receivers. The limited cell area enables the channel frequencies in one cell to be reused in another geographically separated cell according to an established scheme. Thus, a large number of channels can be made available in a metropolitan area and service can appear to be identical to that of a standard telephone. The cellular system typically utilizes one channel in each cell, called a "control channel", to receive requests for service from subscriber units to call other subscriber units and to instruct subscriber units (also referred to herein as "mobile telephone units") to tune to a frequency pair of a selected "voice channel" wherein a two-way (duplex) conversation may take place. The control channel continuously receives and transmits data and is the channel to which a mobile telephone unit automatically tunes when not in a voice conversational state.
Since the cells may be of relatively small size, typically ten miles in radius, the likelihood of a mobile telephone unit traveling out of one cell and into another is high. To maintain uninterrupted communications, the mobile telephone unit is "handed off" between one cell and another. The cell systems track the mobile telephone unit and decide when a "handoff" operation is necessary to maintain high quality communication. The mobile telephone unit is commanded by a high speed data message which interrupts the audio communications on a voice channel to re-tune the transceiver to another frequency that presently is available in a new cell to which the mobile telephone unit is "handed off". This handoff operation requires a relatively short period of time (200 to 700 milliseconds), and the mobile telephone user usually is unaware of the occurrence.
Digital data messages usually are transmitted in a Manchester encoded format, which is well known by those skilled in the art. This digital transmission system has been thoroughly studied for application to high capacity mobile telephone systems. Immunity to radio channel fading and other performance characteristics have been reported. Since cellular telephone systems interconnect with and provide performance that is characteristic of ordinary land line telephone systems, subscribers also tend to expect identical or similar capabilities and characteristics from a cellular telephone system. One such capability is the transmission of digital data via a mobile telephone unit from one location to another. Many telephone subscribers connect data communication devices such as a personal computer (PC) to the telephone system via a modem. Modems are familiar to those skilled in the art and operate by converting digital "one" and "zero" levels to distinct tones or to particular tone waveforms which can be transmitted by the land telephone network.
It would be very desirable for owners of portable personal computers to be able to couple them via a modem, a mobile telephone unit, and a cellular telephone system to another remote computer. Unfortunately, this has been problematical. Various systems have been devised to solve the digital data communication problems caused by the above mentioned "handoff" problem and other automatic operations that require interruption of digital data transmission/reception, but none of the known systems has been completely satisfactory. During handoff or other interruption such as adjustment of transmitter power level, the personal computers of both the mobile telephone unit and the transmitter of the base site controller presently communicating with the mobile telephone unit cause both the mobile telephone unit and the transmitter of the base site controller to halt all digital communication for the above-mentioned 200 to 700 millisecond interval during which the handoff, power adjustment, or other similar operation takes place. The personal computers which may have been communicating during such operation have no way of knowing about the 200 to 700 millisecond delay, and as a result digital data is lost, and also there is an insertion of a burst of up to 463 bits of information and commands onto the voice channel from the control channel during the handoff operation or similar operation.
Pat. No. 4,654,867 (Labedz et al) describes a technique for dealing with this problem by accomplishing digital data transmission between personal computers over the control channels. More specifically, the system described in the Labedz patent requires all data communication between a "local" PC (personal computer) and a "remote" PC to occur at high speed (up to 9600 bits per second) over the control channel of the cellular telephone system. Consequently, complex equipment is needed at each end of the cellular telephone communication link for the purpose of converting the data from low "voice channel" modem speeds to "control channel" speeds of up to 9600 bits per second and then converting the data back down to the low voice channel modem speeds The complex equipment also must change the data format to allow transmission over the control channel, and then must change the data format back to its original format suitable for transmission over the voice channel. This complexity of the Labedz system adds greatly to its cost. Furthermore, that system is only compatible with other identical systems, but not with IBM-compatible and other popular personal computer systems.
Another system is described in Pat. No. 4,697,281 to effectuate data communication between two PC's over a voice channel of a cellular telephone system. That system requires large buffers in units connected to each PC to temporarily store all data that has a possibility of being lost during a handoff, and both units continually compare the contents of their respective buffers to determine if a handoff, power adjustment, or other interference occurred, and if it did, retransmits the data until the two buffers match.
It would be desirable to provide an inexpensive device which can be connected to any personal computer to enable it to communicate digital data over a cellular telephone system to a remote computer, such as an IBM PC or the like, without concern for loss of digital data during handoffs, power adjustments, and other like automatic operations that in the past have interfered with digital data transmission over cellular telephone systems.